A typical oil or gas well has a wellhead housing with a Christmas tree mounted thereon. One or more strings of casing extend into the well and are supported by casing hangers landed in the wellhead housing. In one type of wellhead assembly, a tubing hanger lands in the wellhead housing. The tubing hanger supports a string of production tubing suspended in the casing. Well fluid flows up the tubing to the tree. If the wellhead assembly is a surface installation, rather than subsea, normally the wellhead has a tubing annulus access port extending through its sidewall. The access port is located below the tubing hanger seal to provide access to the tubing annulus.
In some wells a minimum velocity of the well fluid is desired as it flows through the tubing hanger. As the formation is depeleted, the bottom hole pressure declines, causing a reduction in velocity. An inner string of tubing may be installed in the previously installed tubing. In one technique, the tree is removed, and a tubing spool or tubing head is connected to the upper end of the wellhead housing. The inner string of tubing is lowered into the existing tubing, and an inner tubing hanger lands in the tubing spool. The inner string of tubing may be joints of production tubing secured together by threaded ends, or it may comprise a continuous string of coiled tubing. After securing the inner tubing hanger in the tubing spool, the tree is connected to the tubing spool. While workable, this adaptation increases the overall height of the wellhead assembly, which can create problems, particularly for wellhead assemblies mounted on offshore platforms.